A hybrid vehicle refers to a motor vehicle that uses an electric motor and an internal combustion engine as drive power sources. In the case where the drive load of the hybrid vehicle is large, for example, when the hybrid vehicle is driven at a high speed or when the hybrid vehicle is driven on an uphill road, the hybrid vehicle is operated in an engine drive mode in which the internal combustion engine is used.
Conversely, in the case where the drive load of the hybrid vehicle is small, for example, when the hybrid vehicle is driven at a low speed or when the hybrid vehicle is stopped, the hybrid vehicle is operated in a motor drive mode in which the electric motor is used.
In this hybrid vehicle, the minimized use of the engine makes it possible to suppress unnecessary fuel consumption and to reduce emission of an exhaust gas. It is therefore possible to increase the fuel efficiency and to reduce the air pollution.
However, the frequent stop of the engine of the hybrid vehicle limits the operation of an air conditioning system for heating a vehicle room. This leads to a problem in that the vehicle room heating efficiency is sharply reduced.
Specifically, as shown in FIG. 1, if the drive load of a hybrid vehicle is reduced and if the drive mode of the hybrid vehicle is switched from an engine drive mode to a motor drive mode, the engine is stopped (see curve A in FIG. 1). As a result, the temperature of engine cooling water is reduced (see curve B in FIG. 1). Thus, the engine cooling water having a low temperature is supplied to a heater core of an air conditioning system, thereby reducing the temperature of the heater core (see curve C in FIG. 1). Consequently, the temperature of an air injected into a vehicle room is reduced (see curve D in FIG. 1). This reduces the vehicle room heating efficiency and significantly impairs the ride comfort.
In view of this problem, there has been proposed a technique of preventing a vehicle room temperature from being reduced even when a temperature of a heater core is dropped due to the stoppage of an engine. In this technique, as shown in FIG. 2, when the drive mode of a motor vehicle is switched from an engine drive mode to a motor drive mode in which an engine is stopped (see curve A in FIG. 2), an opening angle of a temperature door (see curve E in FIG. 2) is corrected in response to the reduction of an engine cooling water temperature caused by the stoppage of the engine. Specifically, when the engine is stopped (see curve A in FIG. 2) and when the engine cooling water temperature is reduced (see curve B in FIG. 2), the opening angle of the temperature door is corrected in proportion to the reduction of the engine cooling water temperature so that a hot air path is further opened (see curve E in FIG. 2).
Accordingly, the temperature of the air injected into the vehicle room can be increased even when the engine cooling water temperature is reduced due to the stoppage of the engine (see curve F in FIG. 2). Thus, it is possible to compensate the reduction of the vehicle room temperature even when the engine cooling water temperature is reduced due to the stoppage of the engine and even when the heater core temperature is eventually reduced. As a result, it is possible to maintain the vehicle room temperature at a desired temperature regardless of the operation or stoppage of the engine.
In the meantime, as illustrated in FIG. 3, if the engine cooling water temperature is excessively reduced to a predetermined lower limit value during a motor drive mode, the engine is re-operated. Thereafter, if the engine cooling water temperature is increased to a predetermined upper limit value or more, the engine is stopped again. Thus, the engine cooling water temperature can be maintained between the predetermined upper limit value and the predetermined lower limit value. The reason for employing this configuration is to prevent excessive reduction of the engine cooling water temperature, thereby maintaining the heating performance substantially constant.
However, the technique of the related art is configured to correct the opening angle of the temperature door based on the reduction of the engine cooling water temperature in the motor drive mode. For that reason, in the motor drive mode, it is impossible to accurately control the opening angle of the temperature door in conformity with the change in the actual temperature of the air injected into the vehicle room. This makes it impossible to accurately compensate the vehicle room injection air temperature in the motor drive mode.
In the motor drive mode, the change (reduction) of the vehicle room injection air temperature is affected by different factors such as an engine cooling water temperature, a blower air volume, an evaporator temperature, an internal/external air temperature, a solar radiation amount and the like.
Thus, in the technique of the related art in which the opening angle of the temperature door is corrected solely based on the reduction of the engine cooling water temperature, it is impossible to accurately control the opening angle of the temperature door in conformity with the change in the temperature of the air injected into the vehicle room. This makes it impossible to accurately compensate the vehicle room injection air temperature in the motor drive mode. Thus, the vehicle room temperature cannot be maintained at a pleasant temperature in the motor drive mode.
In addition, in the technique of the related art, the engine is automatically re-operated if the engine cooling water temperature is a predetermined lower limit value or less in the motor drive mode. This leads to a problem in that the engine re-operation phenomenon occurs frequently.
Specifically, the engine is automatically re-operated regardless of a target air injection temperature set by a user if the engine cooling water temperature is a predetermined lower limit value or less. For that reason, the engine may be unnecessarily re-operated even when the target air injection temperature is set low and even when the vehicle room injection air temperature can be sufficiently compensated even under an engine cooling water temperature of a lower limit value or less.
Since the engine is automatically re-operated regardless of the opening angle of the temperature door if the engine cooling water temperature is a predetermined lower limit value or less, there is a problem in that the engine may be unnecessarily re-operated even when the vehicle room injection air temperature can be sufficiently compensated by controlling the opening angle of the temperature door without having to re-operating the engine. This poses a problem in that the engine re-operation phenomenon occurs frequently in the motor drive mode, consequently reducing the fuel efficiency.